Quantum Battery Charging Improved

Batteries that exploit quantum phenomena to gain, distribute and store power promise to beat conventional chemical batteries in certain low-power applications.

University of Tokyo and other researchers have takne advantage of an unintuitive quantum process that disregards the conventional notion of causality to improve the performance of so-called quantum batteries.

Quantum batteries are still laboratory experiments, and researchers around the world are working on the different aspects that are hoped to one day combine into a fully functioning and practical application.

Above – Coupling strength dependence of performance for ICO charging protocol, its DCO, and NUCC counterparts. (a) Logarithm for population ratio log(1−p)/pR versus κ/ω. R−∗, R−max, R−min, RDmax, and RN,−∗ are shown by (blue) solid, dashed, dotted, red, and orange curves, respectively. The corresponding experimental results are shown by symbols. (b) Thermal efficiency η versus κ/ω, where the meaning of curves(dots) with distinct colors are similar to those in (a).

A previous group of researchers Gaoyan Zhu and Professor Peng Xue from Beijing Computational Science Research Center, experimented with charging quantum battery using optical apparatuses such as lasers, lenses and mirrors, but the way they achieved it necessitated a quantum effect where events are not causally connected the way everyday things are. Earlier methods to charge a quantum battery involved a series of charging stages one after the other.

The Tokyo team instead used a novel quantum effect they call indefinite causal order, or ICO. In the classical realm, causality follows a clear path, meaning that if event A leads to event B, then the possibility of B causing A is excluded. However, at the quantum scale, ICO allows both directions of causality to exist in what’s known as a quantum superposition, where both can be simultaneously true.

“With ICO, we demonstrated that the way you charge a battery made up of quantum particles could drastically impact its performance,” said Chen. “We saw huge gains in both the energy stored in the system and the thermal efficiency. And somewhat counterintuitively, we discovered the surprising effect of an interaction that’s the inverse of what you might expect: A lower-power charger could provide higher energies with greater efficiency than a comparably higher-power charger using the same apparatus.”

The phenomenon of ICO the team explored could find uses beyond charging a new generation of low-power devices. The underlying principles, including the inverse interaction effect uncovered here, could improve the performance of other tasks involving thermodynamics or processes that involve the transfer of heat. One promising example is solar panels, where heat effects can reduce their efficiency, but ICO could be used to mitigate those and lead to gains in efficiency instead.

Physical Review Letters – Charging Quantum Batteries via Indefinite Causal Order: Theory and Experiment

Abstract

In the standard quantum theory, the causal order of occurrence between events is prescribed, and must be definite. This has been maintained in all conventional scenarios of operation for quantum batteries. In this study we take a step further to allow the charging of quantum batteries in an indefinite causal order (ICO). We propose a nonunitary dynamics-based charging protocol and experimentally investigate this using a photonic quantum switch. Our results demonstrate that both the amount of energy charged and the thermal efficiency can be boosted simultaneously. Moreover, we reveal a counterintuitive effect that a relatively less powerful charger guarantees a charged battery with more energy at a higher efficiency. Through investigation of different charger configurations, we find that ICO protocol can outperform the conventional protocols and gives rise to the anomalous inverse interaction effect. Our findings highlight a fundamental difference between the novelties arising from ICO and other coherently controlled processes, providing new insights into ICO and its potential applications.